1. Technical Field of the Invention
This disclosure generally relates to a voltage regulator, and especially relates to a voltage regulator that is capable of providing an output voltage at a predetermined voltage regardless of a fluctuation of an input voltage.
2. Description of the Related Art
[Background Technique]
In recent years and continuing, for environmental reasons, power saving is required of electrical apparatuses, especially battery-driven apparatuses. Generally, lithium ion batteries are used by the electrical apparatuses such as a notebook PC, a cellular phone, and a PDA (Personal Digital Assistant).
The cut-off voltage (end of discharge voltage) of the lithium ion batteries used to be about 3 V, which is now improved to be 2.5 V or less so that a longer service period is provided before recharging becomes necessary.
Patent Reference 1 discloses a voltage regulator that includes a two-step operational amplifier that is constituted by a differential amplifier, and an output unit arranged on the same semi-conductor substrate by a MOSFET circuit. The voltage regulator further includes a reference voltage generating unit. A reference voltage generated by the reference voltage generating unit is provided to one of input terminals of the differential amplifier. To the other terminal of the differential amplifier is connected an output voltage of the output unit, which output voltage is divided by a feedback resistor. Then, the output of the differential amplifier is provided to an input terminal of the output unit. A power supply voltage of the differential amplifier is less than a power supply voltage of the output unit; specifically, the power supply voltage to the differential amplifier is stepped-up and is provided to the output unit.
Patent Reference 2 discloses an electronic apparatus that realizes power-saving effectiveness by improving power-source effectiveness of a battery. According to the electronic apparatus, the output voltage of the battery is compared with a predetermined voltage; if the output voltage is greater than the predetermined voltage, the output voltage of the battery is converted to a second voltage by an output control unit; if the output voltage is less than the predetermined voltage, the output voltage is stepped up to a first voltage, and then converted to the second voltage by the output control unit.
[Patent reference 1] JPA 2005-339467
[Patent reference 2] JPA 2006-081369
Although the voltage regulator proposed by Patent Reference 1 (JPA 2005-339467) increases the power supply voltage to the differential amplifier by a step-up unit, and the stepped-up voltage is supplied to the output unit, the step-up unit cannot be turned on and off.
Further, as described above, as for the lithium ion battery used by the electronic apparatuses, the cut-off voltage is improved to be less than 3 V. However, there are cases wherein the electronic apparatuses operate at a voltage greater than 3 V, in which case, a voltage output from, the lithium ion battery has to be stepped up. With the electronic apparatus of Patent Reference 2 (WA 2006-081369), the output voltage of the battery is compared with the predetermined voltage. If the output voltage is greater than the predetermined voltage, the output voltage is adjusted to the second voltage by the output control unit; and if the output voltage is less than the predetermined voltage, the output voltage is stepped up to the first voltage, and then converted to the second voltage by the output control unit. Here, the step-up unit is prepared on an IC substrate different from the voltage regulator. For this reason, circuit scale becomes great, circuit configuration becomes complicated, and the electronic apparatus becomes great in size. Further, when the voltage is stepped up for providing power to the electronic apparatus, electric power is consumed by the step-up unit, which degrades the power-source effectiveness of the battery. Furthermore, if a DC-to-DC converter is used as the step-up unit, as proposed by Patent Reference 2, costs are increased and the electric power consumed by the DC-to-DC converter is great.